Storm sewer route survey

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STORM SEWER ROUTE SURVEY. The character of the route survey for a storm sewer depends on the circumstances. The nature of the ground may be such as to indicate, without the necessity for reconnaissance and preliminary location surveys, just where the line must go. This is likely to be the case in a development area; that is, an area that will be closely built up and in which the lines of the streets and locations of the buildings have already been determined. In these circumstances, the reconnaissance and preliminary surveys may be said to be done on paper.

On the other hand, a lineor parts of itoften must be run for considerable distances over rough, irregular country. In these circumstances the route survey consists of reconnaissance, preliminary location, and final-location surveys. If topographic maps of the area exist, they are studied to determine the general area along which the line will be run. If no such maps exist, a reconnaissance party must select one or more feasible route areas, run random traverses through these, and collect enough topo data to make the planning of a tentative route possible.

After these data have been studied, a tentative route for the line is selected. A preliminary survey party runs this line, making any necessary adjustments required by circumstances encountered in the field, taking profile elevations, and gathering enough topo data in the vicinity of the line to make design of the system possible.

The system is then designed, and a plan and profile are made. Figure 10-3 shows a storm sewer plan and profile. The project here is the installation of 230 feet of 18-inch concrete sewer pipe (CSP) with a curb inlet (CI "A"). The computational length of sewer pipe is always given in terms of horizontal feet covered. The actual length of a section is, of course, greater than the computational length because of the slope.

The pipe in figure 10-3 is to run downslope from a curb inlet to a manhole in an existing sewer line. The reason for the distorted appearance of the curb inlet and manhole, which look much narrower than they would in their true proportions, is the exaggerated vertical scale of the profile. The appearance of the pipe is similarly distorted.

The pipe to be installed is to be placed at a gradient of 2.39 percent. The invert elevation of the outflowing 21-inch pipe at the manhole is 91.47 feet; that of the inflowing 18-inch pipe is to be 92.33 feet. Obviously, there is a drop here of 0.86 foot. Of this drop, 0.25 foot is because of the difference in diameters; the other 0.61 foot is probably because of structural and velocity head losses.

From the invert in at the manhole, the new pipe will extend 230 horizontal feet to the invert at the center line of the curb inlet. The difference in elevation between the invert elevation at the manhole and the invert elevation at the curb inlet will be the product of 2.39 (the grade percentage) times 2.30 (number of 100-foot stations in 230 horizontal feet), or 5.50 feet. Therefore, the invert elevation at the curb inlet will be 92.33 feet (invert elevation at the manhole) plus 5.50 feet, or 97.83 feet. The invert elevation at any intermediate point along the line can be obtained by similar computation.

The plan shown in figure 10-3 is greatly simplified for the sake of clearnessit contains the bare minimum of data required for locating the new line. Plans used in actual practice usually contain more information.

The plan and profile constitute the paper location of the line. A final-location survey party runs the line in the field. Where variations are required because of circumstances discovered in the field (such as the discovery of a large tree or some similar obstruction lying right on the line), the direction of the line is altered (after receiving approval to do so) and the new line is tied to the paper location. The final-location party may simply mark the location of the line and take profile elevations, or it may combine the final-location survey and the stakeout (which is part of the construction survey, rather than the route survey) in the same operation.